In order to keep pace with the rapid growth of mobile data consumption, operators are investing billions of dollars in building out 4G networks with small cells co-located with macro cells to form heterogeneous networks. They also are adopting self-organizing network (SON) solutions to manage increasingly complex networks and to solve challenges such as interference coordination between macro cells and small cells, self-installation and configuration of small cells, inbound and outbound mobility support to and from small cells, etc.
SON, in general, refers to self-configuration, self-optimization and self-healing functionality that helps to minimize operation expenditures. Some SON functions that have slow adaptations are implemented in a centralized SON, which is usually located at the network management or element management level. Some other SON algorithms that may perform independently or need fast adaptations are implemented as distributed SON and located at the network element level, e.g., the eNodeB level.
Distributed SON functionality in an eNodeB may make decisions and execute parameter changes independently and quickly. Hence, it requires fast updates of RF environment changes and configuration changes in neighbor eNodeBs. To enable these high-speed updates, the X2 interface has been introduced in the LTE standards to permit direct exchange of information between pairs of eNodeBs. This interface allows the exchange of near-real time information that can be used in many SON functionalities including mobility management, load management, configuration update, mobility robustness optimization and energy savings.
For example, the load information message between eNodeBs over an X2 interface can be used in a distributed SON load balancing scheme. Uplink interference overload information, downlink power scheduling information, and downlink almost blank subframe (ABS) information for time-domain interference coordination can be used to optimize transmit powers of LTE resources. The resource status update messages between eNodeBs provide radio resource, S1 transport network load (TNL), hardware load, and ABS status. Even though these resource status updates can be obtained over a northbound interface, receiving these messages over the X2 interface provides much faster and more frequent updates. Other messages that support distributed SON functions in eNodeBs, such as handover preparation, mobility parameter management, configuration update, radio link failure indication and handover report messages are also reported over the X2 interface.
Even though the X2 interface facilitates the implementation of distributed SON functions, there are other challenges to be resolved for distributed SON. First, parameter changes and some SON decisions executed at one eNodeB may trigger other radio environment changes at neighboring cells and cause parameter changes at these eNodeBs as well. The propagation of parameter changes may lead to a long transition period. Also, note that since decisions are made in a distributed fashion, the parameter changes may not reach a steady state. Second, local optimizations may not ensure overall network optimization. Hence a centralized SON approach may still be used to override radio parameter decision made by distributed SON. Therefore, parameter changes and interference coordination activities at the distributed SON need to be monitored by the centralized SON.